A truss is a framework of members fastened together at their ends to support loads. A plane truss has all members lying in a single geometric plane whereas a space truss has members lying in three-dimensions. Truss members are typically long, straight, slender, and have constant cross-section. They can be fastened by welding, riveting, bolting (often gusset plates are used), or other means. Members support load by tension/compression only for maximum structural efficiency (bending is not as efficient). These robust types of structures provide greater stiffness, strength, and damage tolerance over other structural choices at lower weight and cost, are relatively straightforward to analyze, design, and fabricate, while their exposed interior provides much open space for other functionalities. Skyscraper framing, bridges, roof supports, transmission line towers, radio antennas, etc. are all commonly made from trusses. Classic examples include the Eiffel Tower and cellular geodesic domes.
Honeycombs are cellular solids made from a collection of thin wall open prismatic cells nested together to fill a plane. Exceptionally stiff and strong for their weight, they can also be multifunctional and do much more than just support loads. Aerospace (e.g., aircraft, rockets, spacecraft, etc.) and other industries widely benefit from this form of construction where weight savings is crucial. Some of the many honeycomb applications include sandwich structure cores, impact energy absorbers, flow aligners, filters, insulating panels, radio frequency shields, sound barriers, catalyst support medium, heat exchangers, and acoustic dampeners.
When facing skins are attached to conventional honeycomb cores, the fabrication environment (e.g., humid air, volatile organic compounds, etc.) is trapped within. In space applications, pressure differential between the core and ambient during ascent can lead to mission catastrophic failure. In the near vacuum of space, core release can contaminate sensitive equipment. With time and exposure, the service environment (e.g., humid air) can also be trapped through ingress and diffusion. Aircraft control surfaces, helicopter rotor blades, etc., are all susceptible to moisture accumulation which adds weight, degrades adhesives, accelerates corrosion, steams/freezes, etc. Pressurization/depressurization and heating/cooling cycles (e.g., ground-air-ground) exacerbate the issue. A simple “tap” test sometimes exposes the “dead zones.” One way to repair a wet honeycomb core involves introducing holes into the facing skins, puncturing the cell walls, heating to remove the moisture, and then patching. However, damage caused by the moisture largely remains, steam pressure buildup during heating or patching weakens the panels, the paths for moisture ingress are not resolved, and the problem is likely to repeat. One can fabricate the honeycomb with porous, perforated, slotted, or drilled walls (or facing skins), and/or offset honeycomb layers, but most of these “breathable” versions have limited fluid throughput and cost more. Furthermore, holes or other alterations tend to concentrate stress and structural integrity may suffer.